Lithium-Induced Reorientation of Few-Layer MoS2 Films

Abstract

Molybdenum disulfide (MoS₂) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS₂. However, the influence of lithium on the growth of MoS₂ layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS₂ few-layer films prepared using a new method based on one-zone sulfurization with Li₂S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS₂ films or during MoS₂ formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS₂ films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS₂.

Figure 1

(a) Normalized Raman spectra of undoped MoS₂ films with different thicknesses grown by one-zone sulfurization at 800 °C for 30 min on the c-plane sapphire substrate. Corresponding GIWAXS reciprocal space maps for 4 nm (b), 12 nm, (c) and 40 nm (d) thick MoS₂ films.

Figure 2

Normalized Raman spectra of lithiated MoS₂ films with different thicknesses grown in two steps by one-zone sulfurization at 800 °C for 30 min on the c-plane sapphire substrate with 20% (a) and 50% (b) Li₂S portion. XRD patterns of the same films with 20% (c) and 50% (d) Li₂S portion. Azimuthal φ-scans of 103 diffraction of 4 nm lithiated MoS₂ are shown as insets.

Figure 3

GIWAXS reciprocal space maps of lithiated MoS₂ films with different thicknesses (4, 12, and 40 nm) grown in two steps by one-zone sulfurization at 800 °C for 30 min on the c-plane sapphire substrate with 20% (a, b, and c) and 50% (d, e, and f) Li₂S portion.

Figure 4

Normalized Raman spectra of lithiated MoS₂ films with different thicknesses grown in three steps by one-zone sulfurization at 800 °C for 30 min on the c-plane sapphire substrate from the MoS₂ initial layer with 20% (a) and 50% (b) Li₂S portion. XRD patterns of the same films with 20% (c) and 50% (d) Li₂S portion. Azimuthal φ-scans of 103 diffraction of 4 and 12 nm lithiated MoS₂ are shown as insets.

Figure 5

GIWAXS reciprocal space maps of lithiated MoS₂ films with different thicknesses (12 and 40 nm) grown in three steps by one-zone sulfurization at 800 °C for 30 min on the c-plane sapphire substrate with 20% (a, b) and 50% (c, d) Li₂S portion.

Figure 6

Li 1s and Mo 4s XPS spectra collected from the surfaces of Li-MoS₂ films that were synthesized in two (top) and three (bottom) steps by sulfurization on a c-plane sapphire substrate with a Li₂S portion of 20% (left) and 50% (right) Li₂S portion. All spectra were recorded using a photon energy of 270 eV.

Figure 7

Li K-edge XANES spectra collected on the Li-MoS₂ films prepared by two-step synthesis. The bottom spectrum was acquired on the 40 nm Li-MoS₂ sample after it was dipped into ultra-pure water for a few minutes.

Figure 8

Li K-edge XANES spectra for Li doped 2H-MoS₂ obtained by FEFF calculations. Top: fragments of the structural models for hexagonal 2H-MoS₂ with atoms in tetrahedral and octahedral interstitial sites, substitutionally doped MoS₂, and with lithium near a single sulfur vacancy. Middle: simulated Li K-edge XANES spectra. Bottom: calculated orbital-projected density of states for Li, S, and Mo atoms.

Figure 9

Reflectance (solid line) and transmittance (dashed line) of 12 nm (black) and 40 nm (red) thick Li-doped MoS₂ samples prepared by a two-step method with a Li₂S portion of 20% (a) and 50% (b).